ASP.NET Changing Session-State Modes

I'm attempting to share Session information between PHP and .NET on the same server. The sessions will always be created by the .NET site. From my research I've come to believe that the Session-State Mode "SQLServer" will be perfect for this, as the PHP can check session information directly in the database.

My problem is that the .NET application was developed years ago and uses the InProc Session-State. I have no experience working with ASP.NET. I have no way, that I know of, to test switching from InProc to SQLServer and the site must not go down!

1) Could using web.config to change the Session-State from InProc to SQLServer cause any problems for an old application?

2) Is there a way to test this, such as copying all the files to a test sub-domain?

3) If I bring the site down, is there any reason why I couldn't revert the old web.config and get everything back up.

4) Could using SQLServer instead of InProc cause any issues at all on the .NET side?

5) I also noticed in the IIS for the site there is a UI for changing the Session-States, is it better to use that or web.config (or both)?